1. Field of the Invention
The invention relates to an optical unit for optically scanning a surface, which unit comprises a frame, a radiation source unit, a rotatable mirror unit, and means for forming a scanning spot on the surface.
The invention further relates to an optical scanning arrangement including an optical unit for optically scanning a surface of a medium, particularly an information or recording medium, which unit comprises a frame, a radiation source unit, a rotatable mirror unit, and means for forming a scanning spot on the surface.
2. Description of the Related Art
An optical unit and an arrangement including such a unit are known from U.S. Pat. No. 4,901,297. The known arrangement comprises a magneto-optical apparatus for recording and reading information in/from a recording medium in tape form. In order to form a scanning spot the known apparatus comprises inter alia a laser oscillator, a focusing lens, a polygon mirror which is rotatably supported in a frame, and a correction lens. The known arrangement further includes means for generating a magnetic field during information recording. During recording the recording medium is moved relative to the optical unit and recording tracks are formed on the recording medium in tape form, which tracks are oriented substantially transversely of the direction of movement of this medium. The laser oscillator is mounted on a piezoelectric actuator unit for adjusting the direction of an emitted laser beam so as to keep the scanning spot on a track to be scanned, which is generally referred to as tracking. From an optical point of view, the focusing lens is situated between the laser oscillator and the polygon mirror and is coupled to an actuator unit for focusing the laser beam so as to form a scanning spot situated in a recording or information layer of the medium. Optically, the correction lens is disposed between the polygon mirror and the recording medium.
During use of the known arrangement the laser oscillator is pivoted for the purpose of tracking, pivoting being effected so as to vary the direction of the light beam emerging from the laser oscillator. A disadvantage of the known construction is that the pivoting range is comparatively small owing to the limited field size of the optical system used. As a result, tracking is possible only over a small range, so that only comparatively small speed variations of the recording medium can be corrected. In order to compensate for the limited tracking range the known arrangement requires an accurate speed control for the recording medium. The use of a capstan for this purpose is known per se.
Another disadvantage of the known arrangement is that during scanning of the recording medium both the laser source and the focusing lens have to be moved to permit tracking and focusing. The number of supporting elements and actuators required for this purpose leads to a complex arrangement. Moreover, the required movements of the laser source and the focusing lens demand additional space, which makes the arrangement rather bulky. In addition, the movably arranged laser source gives rise to specific problems with respect to its shielding, its heat dissipation and its electrical connections.